Natural killer cells: innate or adaptive immunity? or both?

The response to a breach of security by an invading organism or immunogen is coordinated by the innate and adaptive arms of the immune system. Recently, the line between the innate and adaptive arms of the immune response has become blurred. Natural Killer (NK) cells were previously thought to be a part of the innate immune response. Mounting evidence of memory in NK cells suggests that these cells may possess characteristics previously thought to be in the domain of the adaptive immune system (see figure Innate and adaptive immunity). Further characterization of the pathways involved in generation of memory NK cells by real-time PCR will further our understanding of the role of these cells in adaptive immunity.

The innate and adaptive immune responses are distinct arms of the immune system that can be categorized based on the ability to learn from a primary exposure. Upon secondary exposure to a pathogen, the cells of the adaptive immune response will exert their effector function in a more rapid, robust way. While the T and B cells of the adaptive immune response are responsible for this long term memory response, the initial immune response is mediated by the cells and factors of the innate immune system. The innate immune response is the first line of defense against infection, and is the most critical arm of the immune response, with defects in this system often resulting in death from lethal infections (1, 2). Components of the innate immune system are conserved from plants all the way through mammals, highlighting their importance for survival.
It was believed that the memory capacity of the immune response was solely mediated by the T and B cells of the adaptive immune response. NK cells were previously thought of as the lymphocytes of the innate immune response due to their lack of recombination-activating gene (RAG) associated antigen receptors and their lymphocyte-like morphology. NK cells are now thought to have memory cell characteristics such as persistence after initial exposure to antigen and a more robust response upon secondary exposure. These observations have now been reported by several labs and in response to multiple pathogens, including herpesvirus, HIV-1, poxvirus, and influenza. This has broad implications for the way we about think adaptive immunity and applications such as vaccine development.

A key feature of the cells of the adaptive immune response is the expression of the RAG genes and their role in the recombination of antigen recognition receptors. This allows for the generation of an infinite number of B and T cell receptors and accounts for a level of specificity in the adaptive immune response. Another feature of adaptive immunity is the four phase program of response to antigen. After foreign antigen is encountered, cells progress through clonal expansion where they mediate effector function. Once the threat is controlled, the immune response must be suppressed. This occurs through a contraction phase, where activated T cells undergo apoptosis, decreasing the overall population by 90-95%. A stable population of memory cells survives, residing in the lymphoid and non-lymphoid tissues, patrolling for reoccurrence of the same antigen. Upon re-exposure, memory cells mount a rapid and robust response (3). This is the basis for vaccination; generation of a population of memory cells poised and ready to spring into action upon meeting their cognate antigen.

The innate immune response is mediated by cells whose antigen recognition receptors are encoded in the germline DNA. They do not require the gene rearrangement that is essential for adaptive immunity. The innate immune response is an ancient part of the host defense system, with common systems found from insects to humans, a well characterized example being the Toll family of receptors. These pattern recognition receptors recognize molecular patterns common to pathogens, such as lipopolysaccharide, a component of the gram negative bacteria cell wall (4).

The cells of the mammalian innate immune system mediate the inflammatory response, triggered through activation of receptors on the cell surface of macrophages, dendritic cells, mast cells, neutrophils, eosinophils and NK cells. These receptors recognize pathogen associated molecular patterns and when activated, rapidly differentiate into short lived effector cells. When the burden becomes too large for the innate immune response to handle, the adaptive immune response is called into action through the activity of antigen presenting cells, such as dendritic cells (DCs). DCs express costimulatory molecules (CD80 and CD86) and can present peptides of pathogen origin in the context of the major histocompatibility complex (MHC) to T cells, activating the adaptive immune response (4).

NK cells can directly kill tumor or virus infected cells, hence their name. They can secrete chemokines that influence their colocalization with dendritic cells in areas of inflammation. They also secrete both inflammatory and immunosuppressive cytokines. Secretion of gamma interferon helps to shape the T cell response, as does direct killing of tumor or infected cells, increasing the amount of antigen available for cross presentation to cytotoxic T cells (5).

NK cells express both activating and inhibitory receptors on their cell surface. In addition to cytokine receptors that are involved in NK cell development and effector function, NK cells also express activating receptors that mediate antibody independent killing of target cells. Mouse NK cells express the Ly49H activating receptor which recognizes a murine cytomegalovirus encoded MHC class I homolog, m157. Another receptor, Np46, has been demonstrated to interact with influenza and parainfluenza derived hemagglutinin proteins. De novo presence of virus specific receptors may have been selected for through co-evolution with these viruses, as these receptors would clearly provide a selective advantage (59).

In recent reports, activation of NK cells has been shown to induce memory cell-like functions (3, 8, 10, 11). These observations call for reevaluation of the role of the NK cell in adaptive immunity.

NK cells were initially shown to be involved in contact hypersensitivity in a mouse model of hapten induced dermatitis (10). Contact hypersensitivity was believed to be mediated by the adaptive arm of the immune system, but was now being observed in mice lacking T cells. This response was shown to be dependent on the presence of NK cells. Adoptive transfer of these hapten exposed cells to naive mice resulted in a delayed type hypersensitivity reaction upon second exposure with the same hapten. This was the first piece of evidence that NK cells may mediate what was previously thought to be an adaptive immune response (10).

Cells stimulated with cytokines have been shown to produce increased levels of gamma interferon upon re-stimulation (11). Cells were stimulated with IL-12 and IL-18 and transferred to a new host after gamma interferon levels declined. Re-stimulation with the same cytokines or engagement of the NK cell receptor resulted in increased gamma interferon production compared to controls. A more robust response upon second exposure is a hallmark of memory cell function.

NK cells can be activated through pathogen specific NK cell receptors, such as Ly49H. Murine Ly49H+ NK cells have been shown to specifically proliferate after infection with murine cytomegalovirus (MCMV). In adoptive transfer experiments conducted by Sun et al, these cells were shown to persist for more than two months after infection (3). The cells demonstrated a more robust response two months after the initial exposure (see figure Generation of NK cell memory). These memory-like cells expressed markers for mature NK cells, low CD27, higher levels of Ly6C, KLRG1, and CD43; however specific markers of memory-like NK cells have yet to be identified (3). Memory NK cells have now been described in mice after exposure to influenza, vesicular stomatitis virus (VSV), and HIV-1 (12). These observations provide evidence of memory-like function in NK cells: clonal expansion, contraction, persistence, and robust secondary response and call for reevaluation of the role of NK cells in innate and adaptive immunity.

The discovery of memory NK cells suggests that these cells may be developmentally and functionally closer to the adaptive immune lymphocytes than previously thought and pose many questions. What is the transcriptional profile of NK cells during the different phases of activation? Are pathways similar to those utilized by T and B cells responsible for generation of memory-like NK cells? Different subsets of memory T cells are generated, expressing different cytokines and homing receptors, does this happen during the generation of memory-like NK cells? Are there central memory and effector memory NK cells? Stable markers need to be identified for identification and differentiation between the different NK cell populations. What are the contributions of stromal elements and other leukocytes to the generation of memory in NK cells? There are many open questions regarding transcriptional control of NK cell differentiation and memory generation. How can these observations be applied to the development of improved vaccines?